The present invention is applicable for use in all cellular radio systems and in so-called WLL (Wireless Local Loop) systems that are interference-limited, and especially in a radio system utilizing code division multiple access. Code division multiple access (CDMA) is a multiple access method, which is based on the spread spectrum technique and which has been applied recently in cellular radio systems, in addition to the prior FDMA and TDMA methods. CDMA has several advantages over the prior methods, for example spectral efficiency and the simplicity of frequency planning.
In the CDMA method, the narrow-band data signal of the user is multiplied to a relatively wide band by a spreading code having a considerably broader band than the data signal. In known test systems, bandwidths such as 1.25 MHz, 10 MHz and 25 MHz have been used. In connection with multiplying, the data signal spreads to the entire band to be used. All users transmit by using the same frequency band simultaneously. A separate spreading code is used over each connection between a base station and a mobile station, and the signals of the different users can be distinguished from one another in the receivers on the basis of the spreading code of each user.
Matched filters provided in the receivers are synchronized with a desired signal, which they recognize on the basis of a spreading code. The data signal is restored in the receiver to the original band by multiplying it again by the same spreading code that was used during the transmission. Signals multiplied by some other spreading code do not correlate in an ideal case and are not restored to the narrow band. They appear thus as noise with respect to the desired signal. The spreading codes of the system are preferably selected in such a way that they are mutually orthogonal, i.e. they do not correlate with each other. In practice the spreading codes are not orthogonal, and the signals of the other users hinder the detection of the desired signal by distorting the received signal. This mutual interference caused by the users is called multiple access interference.
The more simultaneous users there are in the system, the greater the multiple access interference. Therefore the capacity of the CDMA cellular radio system is restricted by the above-described mutual interference caused by the users. The interference can be reduced by keeping the power levels of the signals from the terminal equipments as equal as possible in the base station receiver by means of accurate power control. The base station then monitors the powers of the received signals and transmits power control commands to the terminal equipments.
A known problem in interference-limited systems is the selection of the transmit power used at the beginning of the communication over the connection. A known solution is to start transmitting with a minimum power that is then increased by fixed steps. The disadvantage of this method is that it does not take into account the loading situation of the cell which may vary greatly, but it operates in a similar way in every situation.
In another known method, disclosed in Finnish patent application 952,396 that is incorporated herein by reference, the signal-to-noise ratio is estimated between a subscriber equipment and a base station in the uplink direction before the actual connection is established, and the transmit power is selected so that the desired signal-to-noise ratio is obtained. The disadvantage of this method is that since the terminal equipment starts transmitting directly with a power that is different from the minimum power, the transmission interferes with the other terminal equipments, which in turn must increase their transmit power in order to obtain a sufficient signal-to-noise ratio. In such a case, the total interference in the network increases needlessly.